This invention generally relates to a switching system for controlling communication between transceiving informational sources of a telecommunications network and, more specifically, to such a system in which timing synchronizational information is encoded directly in the digital information being transceived.
In a modern digital telephonic switching system, audio signals between individual subscriber units are PCM encoded and transceived on a time division multiplexing basis. Circuits know as network termination units have means for interfacing with a group of analog or digital telephone lines and segmenting these lines into corresponding group of channels, or time slots, of a time division multiplexing system. The voice information or digital data on any incoming telephone line of the group is assigned to and is successively provided during corresponding ones of a group of time slots for switching to other transceiving units of the system. The data which is in the plural time slots is then provided to a control unit for switching information from any incoming channel to a selected outgoing channel. These control units also contain central processing elements for controlling this switching operation and also provide a central time base for synchronization of the switching operations.
Thus, it can be appreciated that there are several distinct types of information which must be conveyed between each of the control units and their associated network termination unit. The clock reference from the central time base must be transmitted from the control unit to the network termination unit in order to maintain the network termination unit in frequency synchronization with the centrol time base of the control unit. A phase reference must be passed from the control unit to the network termination unit in order to maintain the network termination unit in phase synchronization with the central time base of the control unit. There, of course, must be a voice or data path from the network termination unit to the control unit and vice versa. A control data link must also be provided between the network termination unit and the control unit for both passing messages to the network termination unit to control its operation and, on the other hand, to pass messages from the network termination unit to the control in response to commands from the control unit.
If a separate wire were used for each of the above types of information, six different wires would be required per control unit, network termination unit pair. In a large switching system, even presuming twenty-four channels per network termination unit, the total number of wires, or connections, would be excessive. This fact has become more significant as the size of the individual control and network termination units decreases due to circuit miniaturization which reduces the space available for wire connectors for those units. Accordingly, there is a strongly felt need to reduce the number of wire connections between control and network termination unit pairs to a minimum.
It is, of course, generally known to use time division multiplexing to reduce the number of wires required for coveying different types of information. Referring to FIG. 1A, a data encoding scheme commonly referred to as Manchester coding, or biphase unipolar encoding is also known in which synchronized information is inherently carried by the data being transmitted, i.e. is self-clocking, so a single wire can simultaneously carry both data and clock information. Binary ones and zeros are represented by negative and positive transitions, and, although a transition occurs once during each cycle, whether the transition will be positive or negative is indeterminate, thereby requiring detection for both types of transitions for full clock extraction. Moreover, the transitions occur during the middle of the clock period instead of at the beginning of each cycle. Another scheme known as RZ (return to zero) binary bipolar coding is also self-clocking. However, it undesirably requires a bipolar voltage source, since binary ones and zeros are represented by positive and negative voltage pulses during the start of each cycle which is incompatible with most modern day telephonic switching circuitry. A system known as RZ binary unipolar coding does not require a bipolar source, but it is only partially self-clocking, since pulses do not occur during each clock cycle. A clock detection for such an RZ data source is shown in U.S. Pat. No. 3,894,246 issued Jul. 8, 1973 to Torgrim and assigned to the assignee of this invention. No other coding schemes using a pulse format are known which are self-clocking or which provide for phase synchronization and, thus any known unipolar data line will have to be accompanied by a companion synchronization line.